Tryptophan hydroxylase (TPH) is rate-limiting enzyme in serotonin synthesis of a neurotransmitter importantly involved in numerous behaviors including aggression, reproduction, feeding and depression. In order to further understand the role of serotonergic neurotransmission, we attempted to block the synthesis of TPH by administering directly into the brain, synthetic DNA oligonucleotides that were antisense to mRNA of TPH and control. Control oligo was of the same nucleotide composition but in a scrambled order that had no significant homology to known mRNAs. Male NCI Swiss mice were anesthetized using pentobarbital and cannulated into the IVth ventricle with a modified PE-20 tubing cannula. After 4-5 days of recovery, animals were infused daily with a 4 ml volume of either vehicle (saline), 4 mg antisense oligo, 4 mg scrambled oligo, or 8 mg of parachlorophenylalanine (PCPA), a drug which is known to inhibit TPH. In Experiment One, animals were monitored for core body temperature for 2 days prior to infusion and then daily until completion of the experiment. Animals were infused daily for 3 days and sacrificed on the 4th day. The brain stem was collected and frozen at -70 degrees c until assay. HPLC analysis of TPH activity indicated a significant (greater than 50%) reduction in enzyme activity in antisense oligo and PCPA infused animals as compared to scrambled oligo- and saline-infused controls, which did not differ from each other (ANOVA; p less than 0.05). Analysis of serotonin metabolites indicated changes in the same direction but of a smaller magnitude. There were no consistent effects on core body temperature. Experience Two involved the same procedure except that animals were infused for 5 consecutive days and sacrificed after behavioral testing on the 5 day. Tests of anxiety were suggestive of an effect of PCPA inducing anxiety but there also appeared to be some degree of motor impairment as a result of PCPA treatment. This was not evident in antisense- or scrambled oligo-fused animals. Tissue was harvested and stored until analysis. Specific effects in the absence of motor impairment may prove an important advantage of antisense therapy over drug therapy to modulate serotonin synthesis.